For most new trailers equipped with 25 lb LP tanks on the toungue, can you basically swap them out for 35 lb tanks, or do you need to lengthen the hose and/or lengthen the threaded rod that holds them?

Doesn't seem like rocket surgery to adapt, but curious if you need to adapt anything at all.

First, you are talking about 20 lb (5 gallon) and 30 lb (7 gallon) cylinders. Most of the things you need to make it happen, in addition to the 30 lb. cylinders, themselves:
Make sure there is no clearance problem that hinders installation of the taller cylinders.
Probably longer lines leading from cylinder to regulator. You don't want these lines to be pulled tight.
Longer all-thread center rod to hold them in place while you're traveling.
A fiberglass cover for the taller cylinders.
Otherwise, it is a very simple changeover.

We sprang for aluminum 40 pounders, which weighed about the same full as a steel 30 pounder.
Longer center rod, hard to say about the lp lines, pretty sure I changed the low pressure post regulator line and raised the regulator.